This invention relates to lasers and more particularly to an improved arc detection and control circuit for a high power gas laser.
A CO.sub.2 gas transport laser that has been built and successfully operated for producing continuous wave output power in the order of 1 to 2 KW is described in U.S. Pat. No. 3,772,610. Wide use of that laser and the discovery of new industrial applications have resulted in demands for even higher output powers, i.e., in the order of 5 KW and above. One way to achieve higher power levels in such a laser is to increase the current density in the glow discharge by increasing the current in the discharge. As the current is increased, however, slight variations in the gas flow and composition tend to produce instabilities which cause the laser to arc. The energy contained in such arcs in a high power laser is so high that severe damage to the electrodes, power supply, and other parts of the laser can occur within a relatively short time, i.e., in 50 to 500 milliseconds.
In accordance with prior practice of protecting against arcs in glow discharge devices, a meter relay is connected across the electrodes to detect increases in discharge current occasioned by arcs and to trip a relay when the current exceeds a predetermined level, see "Gaseous Conductors" by James Cobine, pages 205-209 (Dover, 1958). Such a technique is not feasible for the afore-mentioned high power gas transport laser because stable operation of the laser requires that the discharge current be tightly regulated, i.e., held constant. This means that the current does not change substantially when the arc occurs. Instead, the voltage across the electrodes drops. Further disadvantages of the meter relay approach are the slow response time which permits considerable arc damage to the internal laser components prior to automatic shutdown of the power supply and the generally unreliable operation due to the fact that the operator must remember to set the current trip needle just above the operating point, a step which has, in practice, often been forgotten.
This invention is directed to a solution to these problems.